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Re: "x == None" vs "x is None"

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From Chris Angelico <rosuav@gmail.com>
Newsgroups comp.lang.python
Subject Re: "x == None" vs "x is None"
Date Sun, 17 Jan 2016 22:29:57 +1100
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On Sun, Jan 17, 2016 at 10:05 PM, Ulli Horlacher
<framstag@rus.uni-stuttgart.de> wrote:
> Chris Angelico <rosuav@gmail.com> wrote:
>> On Sun, Jan 17, 2016 at 8:51 PM, Ulli Horlacher
>> <framstag@rus.uni-stuttgart.de> wrote:
>> > I have seen at several places "x == None" and "x is None" within
>> > if-statements.
>> > What is the difference?
>> > Which term should I prefer and why?
>>
>> tl;dr: Prefer "x is None" as a check.
>
> And for the negation?
> "if not x is None" or "if x is not None"
>
> I have seen the last one several times, but i do not understand it, because:
>
>>>> x=0
>>>> x is not None
> True
>>>> not None
> True
>>>> x is True
> False

There's no actual difference:

>>> dis.dis(lambda x: x is not None)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE
>>> dis.dis(lambda x: not x is None)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE

The compiler can tell that "not x is None" is exactly the same thing
as "x is not None". Personally, I'd rather spell it "is not None",
partly because it reads more like English that way, and partly because
one operator is better than two; but you're welcome to spell it "not x
is None" if that has other benefits (eg consistency).

ChrisA

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Thread

"x == None" vs "x is None" Ulli Horlacher <framstag@rus.uni-stuttgart.de> - 2016-01-17 09:51 +0000
  Re: "x == None" vs "x is None" Chris Angelico <rosuav@gmail.com> - 2016-01-17 21:15 +1100
    Re: "x == None" vs "x is None" Ulli Horlacher <framstag@rus.uni-stuttgart.de> - 2016-01-17 11:05 +0000
      Re: "x == None" vs "x is None" Peter Otten <__peter__@web.de> - 2016-01-17 12:26 +0100
      Re: "x == None" vs "x is None" Chris Angelico <rosuav@gmail.com> - 2016-01-17 22:29 +1100
      Re: "x == None" vs "x is None" <paul.hermeneutic@gmail.com> - 2016-01-17 11:01 -0700
      Re: "x == None" vs "x is None" Random832 <random832@fastmail.com> - 2016-01-17 16:33 -0500
      Re: "x == None" vs "x is None" Chris Angelico <rosuav@gmail.com> - 2016-01-18 08:38 +1100
      Re: "x == None" vs "x is None" Ben Finney <ben+python@benfinney.id.au> - 2016-01-18 13:46 +1100
  Re: "x == None" vs "x is None" fernando junior <fernandojr.ifcg@live.com> - 2016-01-19 14:20 -0800

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